Leaky coaxial cable

ABSTRACT

This invention provides a leaky coaxial cable suitable for vehicle communication. The leaky coaxial cable of this invention has on the surface of its outer conductor a number of slots which are positioned zigzag at a cycle approaching lambda g/2 ( lambda g: guide wave-length of the coaxial cable) and leaks a part of the electromagnetic energy propagating in the coaxial cable in the form a vertical polarized wave through the slots. This invention further provides a method of manufacturing such a leaky coaxial cable and a vehicle communication system using such a leaky coaxial cable.

0 United States Patent 1191 1111 3,810,186 Nakahara et al. May 7, 1974 LEAKY COAXIAL CABLE 2,816,285 12/1957 Topol 343/770 Inventors: Tsunw Nakahara, Nishimmiya; 5232:31 1: 7 /32? 2223i: I: it Nmwka Kul'auchl, Sulta; Tachlro 3,221,331 11/1965 Spitz 343/785 flga Osaka, all of Japan 3,230,957 l/l966 Seifert 343 771 x [73] Assignee: Sumitomo Electric Industries, Ltd.,

' Przmary Exammer-Arch1e R. Borchelt Osaka, Japan g Asszstant ExammerSaxfield Chatmon, Jr. Flledi 1971 Attorney, Agent, or FirmCarothers and Carothers 21 Appl. No.:- 116,245 ABSTRACT Related U.S. Application Data C f f S N 701 983 J 3 1968 Thls 1nvent1on provides a leaky coaxial cable suitable 8"? 5 0 for vehicle communication. The leaky coaxial cable of a an one this invention has on the surface of its outer conductor a number of slots which are positioned zigzag at a [52] U.S. Cl 343/771, 333/84, 33443477371), Cycle approaching g Z (Ag: guide wavedengfll of the [51] Int Cl Holq 13/10 coaxial cable) and leaks a part of the electromagnetic [58] Fieid 772 776 energy propagating in the coaxial cable in the form a H31 3 vert1cal polarized wave through the slots.

y This invention further provides a method of 5 References Ci manufacturing such a leaky coaxial cable and a UNITED STATES PATENTS vehicle communication system using such a leaky coaxial cable. 3,l06,7l3 10/1963 Murata et al. 343/770 2,756,421 7/1956 Harvey et al. 343/770 8 Claims, 8 Drawing Figures LEAKY COAXIAL CABLE This application is a continuation of Ser. No. 701,983, filed Jan. 31, 1968, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to leaky coaxial cables for use in vehicle communication systems and its manufacturing methods.

2. Description of the Prior Art As leaky coaxial cables, there have already been, for example, (a) type: a coaxial cable having auniform cord continuous opening provided in the outer conductor; (b) type: a coaxial cable whose outer conductor is made of a tape wound helically to provide a herical opening; and (c) type: a cable having a slot array arranged along the outer conductor. The cable of the type a is of a construction which is not'suitable for obtaining a leaky electromagnetic field having electric component in the circumferential direction of the cable.

The cable of type b has shortcomings in that a uniform electromagnetic field strength cannot be obtained in the region in proximity to the cable, and in that manufacturing the cable of type b is difficult, because it is difficult to wind the metal tape on the insulator of the cable to provide a helical opening having a uniform width.

In the case of a leaky coaxial cable of type having slots of the same shape inclined to the axial direction of the cable and provided at intervals of approximately Ag, a uniform electromagnetic field distribution of the leaky wave can be obtained at a far distance. In the proximity region, one to several wavelengths from the leaky cable, however, field distribution shows a variation for about every Ag. Furthermore, the cable of this type radiates a field containing a considerable level of the axial electric field component, which does not contribute to the coupling with the antenna of a vehicle.

In the examples of such leaky coaxial cables heretofore known, a phenomenon takes place due to the interference between the surface wave mode which propagates with the slots as a surface wave line and the transverse electromagnetic wave mode of the coaxial cable, so that the exterior electromagnetic field is remarkably disturbed. That is to say, no consideration has been paid in particular to the distribution of the electromagnetic field in the proximity of the cable in the instance of the leaky helical coaxial cable and the leaky slotted coaxial cable heretofore in use.

As a result of this, the electromagnetic field in the proximity of the cable is remarkably disturbed. A transmission line as such is not desirable for use in avehicle communication system because an antenna is provided closely adjacent the leaky coaxial cable.

BRIEF DESCRIPTION OF DRAWINGS FIG. I is a perspective view of one embodiment of the leaky coaxial cable of the present invention illustrating the use of straight slots in the outer conductor.

FIG. 2 is a perspective view of the leaky coaxial cable illustrating a Z-shaped slot structure in the outer conductor.

FIG. 3 is a perspective view of the leaky coaxial cable illustrating a curved Z-shaped slot structure in the outer conductor.

FIG. 4a is a perspective view illustrating another means of providing the slot structure in the outer conductor of FIG. 1.

FIG. 4b is a plan view of the flattened outer conductor employed in the leaky coaxial cable shown in FIG. 4a.

FIG. 5 is a perspective view illustrating another means of providing a slot structure in the outer conductor similar to that shown in FIG. 1.

FIG. 6a is a perspective view of the leaky coaxial cable of the present invention illustrating yet another means of providing the slot structure shown in FIG. 1.

FIG. 6b is a plan view of the flattened outer conductor employed in the leaky cable shown in FIG. 6a,

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention provides a new improved leaky coaxial cable which removes the drawbacks present with prior ones.

With reference to the accompanied drawing, several embodiments of the present invention will be explained.

In FIGS. 1 6, 1 denotes the central conductor of the cable which may be of a conductor wire, 2 denotes the insulation layer covering said conductor wire I, which may be of a solid, or foamed body, cup-shaped dielectric bodies laid in a series over the central conductor 1 to support it, dielectric tapes helically woulded around the central conductor 1, or of various other constructions.

3 denotes the outer conductor, provided over the insulation layer 2, and 4 denotes a protection layer to protect the outer conductor 3.

Reference numerals 5 and 6 denote two arbitrarily selected adjacent slots provided in the outer conductor 3. These slots have a width sufficiently narrow in comparison to the propagation guide wave length Ag of the coaxial line, and they are symmetrical with respect to a plane normal to the axis of the cable on the outer conductor at the midpoint between said two adjacent slots.

According to the slot structure the present invention, the interval or cycle period P of the periodic slot structure is double the interval between the centers of adjacent slots.

In the present invention, the interval or period P of the periodic slot structure is decided by the following formulas.

g) P Mtg/(x0 A P Zhohg/Ao Ag where A0: wave-length in free space Ag: guide wave-length in the coaxial cable P: cycle or repetitive interval of the periodic slot structure I The interval, P, which satisfies the formulas l and 2, provides one leaky wave component. That is to say, the leaky wave has radiation in one direction determined by the equation 0 sin (ha/P ka/kg), where 0 is a radiation angle measured from a normal to the cable axis in the direction of the cable axis. If P is less than Aohg (A Ag), a leaky wave cannot be generated, and if P is larger than 2A0Ag/(k0+ \g), a plurality of leaky wave components having different radiation angles respectively are generated and interfere with each other, therefore, the external field intensity fluctuates along the cable due to the interference.

On the other hand, investigating the frequency characteristics of the radiation angle 0 by using the equation 6 sin (A0/P- Ito/Ag) with P regulated under the assumption of inequalities l and 2, it is understood that the frequency characteristics of the radiation angle become less variable for a larger value of P because Ao/Ag is nearly independent of frequency. On the other hand, a horizontally polarized external field component can be strongly suppressed when the interval between the adjacent slots (W2) is nearly Ag/Z.

In FIG. 1, narrow and long slots 5 and 6 are of the same length and width and are alternately inclined in the opposite direction at a given angle to the cable axis. They are provided on a side of the cable and the distance P/2 between the centers of adjacent slots is determined to be nearly approaching or (slightly less than) )tg/Z by formulas I and 2.

If the leaky coaxial cable described above is viewed from outside, the slot construction shows an interrupted zigzag line. The two adjacent slots which are separated by approximately kg/2 between their centers are excited respectively by the high frequency currents of the coaxial cable having opposite phase to each other, so that the current flows along the slots inclined alternately in opposite directions and produces an electric field crossing each slot.

If such an electric field is divided into two components in the axial direction and in the direction normal to it, then equivalent axial components are provided in mutually opposed directions such that they effectively or substantially cancel while the components normal to them are directed in the same circumferential direction.

As a result, only the circumferential components remain to effect the radiation of leaky wave.

In the case of vehicle communication where a vertically polarized leaky wave is used, the radiation of the leaky wave from the leaky coaxial cable is required to have only the circumferential electric field component in order to realize an efficient coupling between a leaky cable and an antenna aboard a vehicle and to have low fluctuation of the coupling level.

The cable of the present invention is very suitable for this purpose, because it satisfies the above mentioned requirements.

According to the present invention, a uniform exterior leaky electromagnetic field distribution can be obtained even in a region considerably close to the cable with a leaky electromagnetic field having circumferential electric field components.

This is desirable for leaky coaxial cable to be used in vehicle communication.

Referring to the annexed drawings, further embodiments of the present invention will be explained.

In the embodiment of the present invention shown in FIG. 2 and FIG. 3. Z-shaped narrow slots, instead of linear slots as shown in FIG. I, are positioned such that they are alternately reversed. The distance of each center between adjacent slots is selected to be approximately Ag/Z. The leaky coaxial cables of such construction have properties similar to those of the cable shown in FIG. I with respect to the field of the radiation and the leaky electromagnetic wave.

A manufacturing method of the cable of the present invention will be explained. The cable core which an insulation layer 2 covering a central conductor wire 1 can be made with any conventional method.

In order to construct the outer conductor. a conductor tape is punched to have a slot arrangement as mentioned above. This punched tape is attached longitudinally to the cable core insulation and is wrapped therearound to form a coaxial cable. Thereafter, a protection layer ofinsulation tape or of insulation material extruded from a extruder, is applied over the formed outer conductor.

Another embodiment of the present invention is explained in regard to the construction of the outer conductor shown in FIG. 4(a).

In FIG. 4(0), 1 denotes the central conductor, 2 the insulation layer, 3 the outer conductor, 4 the protection layer. Reference numerals 5 and 6 denote the slots mutually inclined in the opposite direction, and 7 the metal binder. A zigzag shaped tape 3 is attached longitudinally to the conventional cable core and wrapped therearound to form a zigzag gap between edges of the zigzag tape. Then, a metal binder 7 is provided at each reversal point of the slot to form the adjacent slots 5 and 6 which are inclined in the opposite direction from each other at a given angle. As a result, a leaky coaxial cable as shown in FIG. 4(a) is obtained, and it possesses properties similar to those of the cable shown in FIG. 1.

The cable shown in FIG. 5 is manufactured by the use of two long and thin metal tapes 8 which are attached to the crests and bottoms respectively of the zigzag gap of the outer conductor shown in FIG. 4(b) to form the adjacent slots 5 and 6 which are inclined in the opposite direction from each other at a given angle.

In the example of the cable shown in FIG. 6, the zigzag metal tape is provided with protrusions 9 as shown in FIG. 6(b) and is attached longitudinally to a cable core and wrapped therearound to form a zigzag gap between the edges of the zigzag tape which is bridged by the protrusions 9 at reversal intervals to provide slots 5 and 6 which are inclined in opposite directions from each other at a given angle. As a result, a leaky coaxial cable as shown in FIG. 4(a) is obtained, and it possesses properties similar to those of the cable shown in FIG. 1.

Many modifications of the embodiment of the present invention may be obtained easily in accordance with the technical idea basic to the present invention.

We claim:

1. A leaky coaxial cable having a central conductor and an outer coaxial sheath typeconductor insulated from said inner conductor, at least one longitudinally adjacent pair of identical but symmetrically opposed non-resonant slots in said otherwise imperforate outer conductor having portions thereof extending in the axial and circumferential directions, said opposed slots having a center-to-center spacing P/2 wherein P is given by the formula M) Ag/(Ao-l-Ag) P 2 A0 Xg/(Ao Ag), wherein A0 is the wave length in free space and Ag is the wave guide wave length in the coaxial cable, to provide a gradient polarized wave when energized having the axial components thereof effectively cancelled.

2. The leaky coaxial cable of claim 1 characterized by a plurality of said slot pairs in said outer conductor forming a longitudinal series of identical but alternately opposed slots.

3. The leaky coaxial cable of claim 2 wherein said slots are inclined with respect to the axis of the transmission line.

4. The leaky coaxial cable of claim 2 wherein said slots have segments which are not inclined to the axis of the transmission line.

5. The leaky coaxial cable of claim 2 wherein said slots have a Z-shaped configuration.

6. A leaky coaxial cable having a central conductor and an outer coaxial sheath type conductor insulated from said inner conductor, a longitudinal series of slots in said outer conductor, adjacent of said slots being identical but symmetrically opposed and having a center-to-center spacing approaching Ag/Z said slots having curved portions to provide a gradient polarized wave when said coaxial cable is energized having the axial components thereof effectively cancelled.

7. A leaky coaxial cable having a central conductor and an outer coaxial sheath type conductor insulated from said inner conductor, at least one longitudinally adjacent pair of identical but symmetrically opposed slots in said outer conductor having portions thereof extending in the axial and circumferential directions, said opposed slots having a center-to-center spacing approaching )tg/Z to provide a gradient polarized wave when energized having the axial components thereof effectively cancelled, said outer sheath type conductor consisting of an elongated conductive strip coextending with said inner conductor and wrapped circumferentially in its transverse direction about insulation means positioned over said inner conductor such that the resulting strip seam runs longitudinally with said central conductor, said seam providing a gap having a uniform zigzag configuration with conductive means covering its points of reversal to provide said slots.

8. The leaky coaxial cable of claim 7 characterized in that said conductive means consists of lateral protrusions of said conductive strip positioned at said points of reversal to bridge said gap. 

1. A leaky coaxial cable having a central conductor and an outer coaxial sheath type conductor insulated from said inner conductor, at least one longitudinally adjacent pair of identical but symmetrically opposed non-resonant slots in said otherwise imperforate outer conductor having portions thereof extending in the axial and circumferential directions, said opposed slots having a center-to-center spacing P/2 wherein P is given by the formula lambda o lambda g/( lambda o+ lambda g) <P< 2 lambda o lambda g/( lambda o + lambda g), wherein lambda o is the wave length in free space and lambda g is the wave guide wave length in the coaxial cable, to provide a gradient polarized wave when energized having the axial components thereof effectively cancelled.
 2. The leaky coaxial cable of claim 1 characterized by a plurality of said slot pairs in said outer conductor forming a longitudinal series of identical but alternately opposed slots.
 3. The leaky coaxial cable of claim 2 wherein said slots are inclined with respect to the axis of the transmission line.
 4. The leaky coaxial cable of claim 2 wherein said slots have segments which are not inclined to the axis of the transmission line.
 5. The leaky coaxial cable of claim 2 wherein said slots have a Z-shaped configuration.
 6. A leaky coaxial cable having a central conductor and an outer coaxial sheath type conductor insulated from said inner conductor, a longitudinal series of slots in said outer conductor, adjacent of said slots being identical but symmetrically opposed and having a center-to-center spacing approaching lambda g/2 said slots having curved portions to provide a gradient polarized wave when said coaxial cable is energized having the axial components thereof effectively cancelled.
 7. A leaky coaxial cable having a central conductor and an outer coaxial sheath type conductor insulated from said inner conductor, at least one longitudinally adjacent pair of identical but symmetrically opposed slots in said outer conductor having portions thereof extending in the axial and circumferential directions, said opposed slots having a center-to-center spacing approaching lambda g/2 to provide a gradient polarized wave when energized having the axial components thereof effectively cancelled, said outer sheath type conductor consisting of an elongated conductive strip coextending with said inner conductor and wrapped circumferentially in its transverse direction about insulation means positioned over said inner conductor such that the resulting strip seam runs longitudinally with said central conductor, said seam providing a gap having a uniform zigzag configuration with conductive means covering its points of reversal to provide said slots.
 8. The leaky coaxial cable of claim 7 characterized in that said conductive means consists of lateral protrusions of said conductive strip positioned at said points of reversal to bridge said gap. 